The present invention concerns a process and a device for depositing, by electron cyclotron resonance plasma, films of carbon nanofibre webs.
In addition, the invention concerns the films of web obtained in this manner.
The technical field of the application may be defined, in a general manner, as that of depositing films of carbon on a substrate.
Such films are, in particular, films of carbon that emit electrons, but we have also sought to develop processes whose purpose is to synthesise films of diamond, and profitably employ the mechanical, optical and electrical properties of diamond at temperatures generally between 400xc2x0 C. and 1,000xc2x0 C., or in order to make DLC (xe2x80x9cDiamond Like Carbonxe2x80x9d) type carbon films, generally at low temperature (20xc2x0 C. to 400xc2x0 C.) and with a high level of Cxe2x80x94C sp3 bonding, in particular for their mechanical properties.
Such films are principally amorphous.
More precisely, the present application is particularly concerned with the preparation of carbon films formed of nanotubes or nanofibres.
Table 1, at the end of the description, shows different devices and processes for depositing, under vacuum, carbon films, used mainly for depositing emissive carbon.
This table highlights two different categories of deposition processes.
The first category involves CVD processes (xe2x80x9cChemical Vapour Depositionxe2x80x9d) in which a gas of organic molecules (often methane) is introduced as a mixture, with or without hydrogen, into a device that enables the Cxe2x80x94C, C-H and Hxe2x80x94H bonds to be broken by electron impact with, for example, the use of a hot filament, the introduction of microwave power, the use of a radio frequency (RF) polarisation or the use of an electron cyclotron resonance (ECR).
Depending on the device used, the operating pressure is either high (filament, microwave, radio frequency) or low (ECR, RF). The result is a dissociation and an ionisation of the particles, which increases as the pressure decreases. The energy that needs to be supplied for the reaction that transforms the gas into a solid is considerably decreased by the breaking of the covalent bonds (for example CH4) of organic molecules.
It is thus possible to obtain graphite or diamond type crystallised structures at lower substrate temperatures (for example, 400xc2x0 C. instead of 800xc2x0 C.). The polarisation of the substrate also makes it possible to favour crystallisation at lower temperature, thus enabling the use of a wider variety of substrates.
The second category of deposition processes groups together the processes, called PVD processes (Physical Vapour Deposition), which involve the direct deposition of carbon atoms or ions, which may be achieved by spraying a graphite target by arc, by laser ablation, by a beam of ions or by evaporation.
The quality and the structure of the films, at a given temperature, mainly depend on the energy of the incident carbon ions or atoms.
In the case of the preparation of carbon nanotube or nanofibre films, which is of particular interest to us, within the scope of the present application, the PVD and CVD processes described above are also used.
Document (18) xe2x80x9cHigh yield of single-wall carbon nanotubes by arc discharge using Rh-Pt mixed catalystsxe2x80x9d, Y, Saito, Y. Tani, N. Miyagawa, K. Mitsushima, A. Kabuya, Y. Nishina (Chemical Physics Letters 294 (1998), Pages 593-598) and Document (19) xe2x80x9cHelical microtubes of graphitic carbonxe2x80x9d S. Iijima (Nature, vol. 354,6 Nov. 1991) pages 56 and 58) concern processes for producing carbon nanotubes by PVD processes, with a direct supply of Cxc2x0 carbon atoms by laser ablation or electric arc. Document (18) describes, more precisely, a process for preparing carbon nanotubes using evaporation by arc between two graphite electrodes in helium, at high pressure (50-1 520 torrs). Binary mixtures of metals from the platinum group, such as rhodium and platinum, are used as catalysts.
The object of document (19) is the synthesis of tube type carbon structures in the form of pins by evaporation by arc discharging with a carbon electrode, within an enclosure filled with argon at 100 torrs.
The nanotubes or nanofibres can also be prepared by CVD processes, via catalytic de-hydrogenation of organic molecules such as acetylene or methane.
The device used may be make use of hot filaments, a radio frequency system or the injection of microwaves at high pressure, which generates atomic hydrogen and radicals or ions, such as CH3+, CH30, CH0, etc.
However, it should be pointed out that forming truly organised architectures of carbon nanofibres or nanotubes and not random and unorganised deposits has been explored up to now.
Document (15) xe2x80x9cLarge-Scale Synthesis of Aligned Carbon Nanotubesxe2x80x9d, W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou, W. Y. Zhou, R. A. Zhao, G. Wang (Science, vol. 274-6. December 1996, pages 1 701-1703) describes the synthesis of aligned carbon nanotubes using a process based on PECVD (Plasma Enhanced Chemical Vapour Deposition) of carbon from the decomposition of acetylene from a gaseous mixture of acetylene and nitrogen, with the deposition being catalysed by microparticles of iron imprisoned within the porous silica that forms the substrate.
Images obtained by scanning electron microscope show that the nanotubes are markedly perpendicular to the surface of the silica and form rows of tubes separated from each other by around 50 micrometers length and spaces of around 100 manometers.
Document (16) xe2x80x9cGrowth of Highly oriented Carbon nanotubes by plasma-enhanced hot filament chemical vapour deposition, Z. P. Huang, J. W. Xu, Z. F. Ren, J. H. Wang, M. P. Siegal, P. N. Provencio (Applied Physics Letters, vol. 73, number 26,28 December 1998 pages 3845-3847) also describes the growth of orientated carbon nanotubes on monocrystalline and polycrystalline nickel substrates by the PECVD process, by using a hot filament. The carbon nanotubes have diameters of 10 to 500 nm and a length of 0.1 to 50 micrometers. Acetylene is used as the carbon source and ammonia is used as the diluting gas and for the catalysis.
xe2x80x9cElectron Field emission from phase pure nanotube films grown in a methane/hydroen plasma, O. M. Kuttel, O. Groening, Ch. Emmenegger, L. Schlapbuch (Applied Physics Letters, vol. 73, number 15,12 October 1998, pages 2 113-2 115) concerns the growth of films of carbon nanotubes on silicon substrates by CVD, from a mixture of methane and hydrogen, using a microwave plasma at a substrate temperature of 900xc2x0 C. to 1 000xc2x0 C. Iron or nickel is deposited, beforehand, on the substrate in order to act as a catalytic seed for growing the nanotubes.
None of the processes described above allow organised architectures of carbon nanofibres or nanotubes to be made with strong bonds between the tubes in order to form a spider""s web (2D structure).
We have seen that the alignment of the nanofibres or nanotubes could certainly be obtained (15) (16), but that, unless particular precautions are taken, the carbon nanotubes often develop (17) in a random unorganised manner, in the form of a jumble of filaments or spike structures without Cxe2x80x94C bonds between the tubes (1D structure).
Although attempts, aiming to develop interconnections, have been carried out (18) by adding nanograins of catalyst, one again obtains, in this case, a disordered and random structure without strong Cxe2x80x94C bonds between the tubes.
In addition, none of the processes described above allow films of nanotubes to be prepared and, moreover, organised architectures of carbon nanofibres or nanotubes, such as webs of nanofibres or nanotubes directly from organic molecules and without a catalyst.
Finally none of the processes allows the deposition of nanofibres or nanotubes over a large surface, in other words generally greater than or equal to 1 m2.
There is therefore a need for a process for depositing webs of carbon nanofibres or nanotubes, not requiring a catalyst, which allows the deposition of such nano-architectures over large areas at a relatively low temperature.
The aim of the present invention is thus to provide a process for depositing webs of carbon nanofibres or nanotubes that meets, amongst other things, all of the requirements mentioned above.
The aim of the present invention is also to provide a process for depositing webs of carbon nanofibres or nanotubes that does not have the disadvantages, defects and limitations of the processes of the prior art and which resolve the problems of the prior art.
This aim and others are achieved, according to the present invention, by a process for depositing a web of carbon nanofibres or nanotubes onto a substrate by electron cyclotron resonance plasma, in the absence of a catalyst, by the injection of a microwave power into a deposition chamber comprising a magnetic structure with a highly unbalanced magnetic mirror, and at least one electronic cyclotron resonance zone within the said deposition chamber and opposite the said substrate, in which, at a pressure less than 10xe2x88x924 mbar, the ionisation and/or the dissociation of a gas containing carbon is induced in the said magnetic mirror at the centre of the deposition chamber, thus producing species that are deposited on the said substrate, which is heated.
More precisely, the said process comprises the following steps:
Heating the substrate
Establishing a pressure equal to or less than 10xe2x88x924 mbar of a gas containing carbon
Injecting microwave power, and creating a plasma from the said gas containing carbon, for a magnetic field value corresponding to the electron cyclotronic resonance
Creating a potential difference between the plasma and the substrate
Dissociating and/or ionising the molecules in the said magnetic mirror at the centre of the deposition chamber
Depositing the species formed on the said substrate in order to obtain the webs of carbon nanofibres or nanotubes
In a particularly advantageous embodiment of the invention, the steps are carried out at the same time.
In fact, the process according to the invention can be placed between the two extreme processes, namely PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposition), and it constitutes an excellent compromise between these two techniques, without having any of the disadvantages.
The process according to the invention meets the requirements mentioned above and resolves the problems of the processes of the prior art and, in particular, unlike the processes for depositing nanofibres or nanotubes according to the prior art, the process according to the invention, which uses a specific ECR plasma, enables deposits to be formed on very large surfaces, greater than, for example, 1 m2.
In the process according to the invention, a source of specific ECR plasma is used, which is a confining source, due to the implementation of a magnetic structure with a highly unbalanced magnetic mirror.
Furthermore, the ECR electronic cyclotron resonance zone, unlike most ECR plasma processes as described in Document (20) xe2x80x9cElectron cyclotron resonance plasma in source for material depositionsxe2x80x9d, M. Delaunay and E. Touchais (Review of Scientific Instruments, vol. 69, number 6, June 1998, pages 2320-2324), is located, according to the invention, within the interior of the deposition chamber itself, opposite the substrate, and is thus integrated with it, and there is therefore no separation between the ECR plasma reaction chamber and the deposition chamber.
Then, in the process according to the invention, the said ECR plasma source, specific and confining, is used at very low pressure, generally less than 10xe2x88x924 mbar.
It is essentially the combination of this specific and confining ECR plasma source with this very low pressure that makes it possible, in the process according to the invention, to strongly dissociate the organic molecules in order to obtain films formed from webs of carbon nanofibres or nanotubes, or networks of interconnected carbon nanofibres or tubes, as in a spider""s web.
The ECR plasma, created according to the invention, is a stationary, stable plasma that, once installed, persists and stabilises itself. Complete dissociation of the molecules is obtained, going up to the end of any possible dissociation. For example, methane can dissociate to give Cxc2x0 species.
More precisely, it can be said that it is the lifetime of the plasma particles that increases and not the lifetime of the plasma.
In other words, this notable magnetic confinement makes it possible to increase the lifetime of the ions and the electrons that remain trapped in the magnetic mirror at the centre of the deposition chamber and along the field lines. The level of dissociation and ionisation of the molecules in the plasma is thus increased with the following types of electron collisions:
CH4+exe2x86x92CH3++H0+e+e
CH4+exe2x86x92CH30+H0+e
CH0+exe2x86x92C0+H0+e, etc.
The low pressure increases the energy of the electrons and reduces re-combination.
The deposited carbon is generally a graphite type carbon with a minority proportion of sp3 bonds and a majority proportion, for example greater than 80%, of sp2 bonds.
In a surprising manner, according to the invention, it has been noted that for a pressure that does not conform with the invention, in other words a pressure greater than 10xe2x88x924 mbar, there is practically no growth of such carbon films but, on the other hand, the appearance of grains of graphite or diamond of various dimensions with, sometimes, cauliflower or lava cluster morphologies.
Films of carbon fibres, with the structure described above and without a catalyst, have never been obtained by the processes of the prior art.
Moreover, and according to an essential advantage of the process according to the invention, the structures of the web like films are obtained directly from an organic compound, without requiring a catalyst, such as a metal, for example nickel, cobalt or another metal.
The magnetic structure with a highly unbalanced magnetic mirror, according to the invention, is such that the magnetic mirror field is maximum (Bmax) at the microwave injection, then the magnetic field is minimum (Bmin) at the centre of the deposition chamber, and finally the magnetic field increases on the substrate (Bsubstrate).
In other words, it involves a strong magnetic mirror upstream at the injection and a weaker magnetic mirror downstream, in other words at the substrate level.
Advantageously, according to the invention the mirror ratio upstream, at the microwave injection, defined by r1 =Bmax/Bmin is greater than 4.
This high mirror ratio makes it possible to make ionised particles (ions and electrons) diffuse towards the substrate under the effect of a decreasing gradient.
Advantageously, the mirror ratio, downstream towards the substrate, defined by r2 =Bsubstrate/Bmin is greater than or equal to 1.5, for example equal to around 1.5.
Preferably, according to the invention, the substrate is heated to a temperature of 500xc2x0 C. to 750xc2x0 C., and preferably 550xc2x0 C. to 700xc2x0 C., in order to provide the activation energy required for growth.
The substrate may be heated by electron bombardment or external heating; the electrons are those of the plasma, attracted by the substrate.
The substrate may be chosen from a wide variety of materials, whose deformation temperatures are greater than the operating temperature such as, for example, glass or silicon.
The substrate does not need to be a conductor. Whatever the case, it is the conductive carbon film that establishes the applied potential.
According to the invention, the pressure is maintained, preferably, at less than or equal to 8.10xe2x88x925 mbar, in order to increase the energy of the electrons and reduce re-combination.
The gas containing carbon may contain carbon in any form whatever; any organic molecule is acceptable.
According to the invention, the gas containing carbon is, for example, methane, ethane, ethylene, acetylene and mixtures, possible supplemented with hydrogen, in any proportions.
Advantageously, the injection of the microwave power takes place at a frequency of 2.45 GHz, at a magnetic field value of B, corresponding to the ECR, around 875 Gauss for a methane type gas.
Generally, the substrate is positively polarised, for example from +20 volts to +100 volts, with a flow of electrons that favours growth without a catalyst, in accordance with the invention, with the plasma connected to frame.
Another possibility consists in polarising the plasma negatively, for example from xe2x88x9220 volts to xe2x88x92100 volts, with the substrate connected to frame.
The invention also concerns a device for electron cyclotron resonance plasma deposition of films of carbon nanofibre webs onto a substrate free of any catalyst, the said device comprising:
a deposition chamber
the means for creating a magnetic structure with a highly unbalanced magnetic mirror in the said deposition chamber
an electron cyclotron resonance zone within the interior of the said deposition chamber and opposite the said substrate
the means for injecting a microwave power into the said deposition chamber
the means for creating a pressure less than 10xe2x88x924 mbar of a gas containing carbon within the interior of the said deposition chamber.
The device according to the invention may comprise, in addition, means for heating the substrate, if the substrate is not sufficiently heated by the electrons of the plasma bombarding the substrate.
The device according to the invention may also comprise, in addition, the means for creating a potential difference between the plasma and the substrate.
As has already be pointed out, the device according to the invention stands out from ECR plasma devices as in Document (20) mainly by the fact that there is no separation between the plasma creation chamber, the diffusion, and the deposition chamber, since the ECR zone is integrated within the deposition chamber.
The invention concerns, in addition, a film, which may be on a substrate, formed of a web or network of interconnected carbon nanofibres or nanotubes, like a spider""s web, the said film being, moreover, free of any catalyst.
This type of web like film structure has never been obtained by processes of the prior art and may be prepared, for the first time, by the process of the invention without a catalyst, due to the use of a specific, confining ECR plasma source, and a low pressure of less than 10xe2x88x924 mbar.
In other words, according to the invention, mono-architectures of carbon fibres or tubes are formed, which may be defined as webs (xe2x80x9cnanowebsxe2x80x9d).
Unlike nanotubes created from catalytic nanograins, notably from metals, such as nickel, cobalt, etc., one obtains, according to the invention, without using any catalyst and, in a surprising manner, networks of interconnected carbon nanofibres, such as in a spider""s web.
The structure of the films according to the invention is an ordered fibrous structure and not a random, unordered structure, as with the prior art, where the films are, besides, polluted and contaminated by the catalyst.
xe2x80x9cFree of catalystxe2x80x9d is taken to mean that the films, according to the invention, do not include elements that could be defined as catalysts, these elements being mainly metals, such as nickel, cobalt, iron, or that these elements are present in trace quantities or as normal impurities.
More precisely, the films according to the invention are made up of nanosegments of carbon linked between each other by strong carbon bonds, which constitutes a different morphology to the carbon nanofibre or nanotube structures of the prior art.
According to the invention, the deposited carbon is graphite type carbon with a minority proportion of sp3 bonds and a majority proportion of sp2 bonds, for example greater than 80%.
xe2x80x9cNano-architecturexe2x80x9d or xe2x80x9cnanowebxe2x80x9d is generally taken to mean that the webs or networks of films according to the invention have an average mesh size of one or several tens of nm to several hundreds of nm, for example from 20 to 200 nm.
Preferably, the average mesh size is 100 nm.
In the same way, nanotubes or nanofibres is generally taken to mean that the diameter of the fibres or tubes is from one to several nm up to one or several tens of nm, for example from 1 to 100 nm, and preferably 20 nm.
The mesh size of the carbon fibre webs increases when the pressure of the gas, such as methane, is reduced, for example from 8.10xe2x88x925 to 6.10xe2x88x925 mbar.
The thickness of the films according to the invention is generally one or several nm to one or several tens of nm, for example from 1 to 100 nm.
The invention also concerns a structure with several layers (multi-layer structure) comprising at least two films of carbon nanofibre or nanotube webs according to the invention, which may be on a substrate.
This type of structure may comprise as many films as required by the application, and may have a thickness generally of from one or several tens of nm to one or several hundreds of nm, for example from 2 to 200 nm.
The structure on which the film or multi-layer structure described above is formed may be chosen from any of the substrates mentioned above; it could, for example, be glass, such a borosilicate glass or silicon.
It should be noted that, by extension, the films or structures according to the invention could also be called xe2x80x9cwebsxe2x80x9d.
The films according to the invention, formed from a web or network of carbon nanofibres or nanotubes have, in addition, apart from their specific structure, a certain number of excellent properties, which make them particularly suitable for a wide range of applications.
The web or fibre of carbon according to the invention is an electrical conductor and refractory, like graphite.
Thus, it generally withstands temperature greater than 700xc2x0 C.
In addition, the mechanical strength of these films is excellent and these films are electron emitters under a field effect, at a field threshold of 10 to 20 V/xcexc.
Moreover, the films are chemically inert at ambient temperature, like graphite.
Finally, for the first time, large areas of films or multi-layer structures, for example from 0.25 m2 to 1 m2 may be formed according to the invention, and without the use of catalysts.
The properties described above may be used to good benefit in numerous possible applications of films and multi-layer structures according to the invention, formed from a web or network of interconnected carbon nanofibres or nanotubes, like in a spider""s web.
The invention thus concerns a filter, in particular a bacterial filter, or a virus filter, comprising at least one of the said films or the said multi-layer structures, which may be on a substrate or on a grid.
In fact, the average mesh size of the carbon nanofibre webs according to the invention corresponds to the best known bacterial filters. For further details, the reader may refer to the work by G. LEYRAL, J. FIGARELLA and M. TERRET, Microbiologie Appliquxc3xa9, volume 2, J. LANORE publisher, p. 150 (liquids) and p. 174 (gases).
In particular, in the case of liquids, filters that can be used for the sterilised filtration of particles and bacteria whose size is greater than 0.2 xcexc can be made, 0.2 xcexc corresponding to the size of the smallest bacteria.
The filters according to the invention will therefore be defined as bacterial filters. For certain films, the filters according to the invention could filter viruses.
The films or multi-layer structures according to the invention may, in an advantageous manner, have a large surface. This property is particularly profitably employed in filtration, where large filtration surfaces have to be available for use.
In the filters according to the invention, the film or multi-layer structure is spread out on a rigid mesh, for example a metal mesh, with larger mesh size, for example several hundreds of xcexc, in order to allow filtration.
The invention also concerns electron accelerator or decelerator nanogrids comprising at least one film or at least one multi-layer structure according to the invention. The conductive and refractory properties of the films according to the invention are exploited in such nanogrids.
Moreover, the invention concerns a flat screen, in particular a large size flat screen, which comprises at least one film or at least one multi-layer structure according to the invention, which may be on a substrate. The fact that these films or structures emit electrons by field effect and can thus advantageously replace the metallic microdots presently used in flat screens may thus be exploited.
The applications given above are only some examples of the applications of the films and structures according to the invention, which may be applied in all fields where their properties, in particular their mechanical strength, may be profitably employed.